Re: [PATCHES] xml2 contrib patch supporting default XML namespaces - Mailing list pgsql-hackers

On 3/6/07, Mike Rylander <mrylander@gmail.com> wrote:
> On 3/6/07, Peter Eisentraut <peter_e@gmx.net> wrote:
> > Mike Rylander wrote:
> > > The patch adds support for default XML namespaces in xml2 by providing
> > > a mechanism for supplying a prefix to a named namespace URI.
> >
> > How does it support multiple namespaces in one document?
>
> It supports one default (unprefixed) namespace URI per document, which
> ISTM is the overwhelmingly common case (and the itch that I must
> scratch).

I think there is some confusion about what the current xml2 contrib
module supports and what my patch adds.  The current code, as it
stands today, supports multiple namespaces just fine.  The only
requirement is that each namespace have a prefix, or else one is
forced to use the local-name() construct with every single node for
those nodes in unprefixed ("default") namespaces.  This patch simply
adds support for registering a prefix for an unprefixed namespace,
which is an extremely common case in XML and causes the use of overly
verbose contortions when designing XPath expressions.  To illustrate
this, xml2 currently supports all of these statements:

SELECT xpath_nodeset('<x><y>foo</y></x>','/x/y');
SELECT xpath_nodeset('<x><a:y xmlns:a="uri:for:a">foo</a:y></x>','/x/a:y');
SELECT xpath_nodeset('<b:x xmlns:b="uri:for:b"><a:y
xmlns:a="uri:for:a">foo</a:y></b:x>','/b:x/a:y');

All number and manner of /prefixed/ namespaces work fine today.
However, in order to match an element or attribute with an unprefixed
namespace, the xpath becomes a study in overly verbose, human error
inducing repetition.  For instance, consider the extremely common case
of an xhtml document that does not use a prefix for the xhtml
namespace.  Using the xml2 contrib module as it stands today, without
my patch, using XPath to get the title of the document might look
something like this:

/*[local-name()="html"]/*[local-name()="head"]/*[local-name()="title"]

Now just imagine the XPath needed to get a portion of the body in a
nested div based on the existence of some other node ... the logic
gets lost in the noise simply because of the overhead of
namespace-qualifying the elements.

Namespaces were introduced in XML to address verbosity issues (among
other things), but as XPath was designed primarily as a language for
use inside XSLT (where namespace support is fully integrated) it
didn't get the treatment needed to handle unprefixed namespaces.  To
address /that/ issue, my patch allows the registration of a supplied
prefix for a supplied URI, which solves the common "default namespace"
problem in a completely backward compatible way.  The above example
XPath can now become:

/x:html/x:head/x:title

simply by supplying 2 more arguments to the _ns version of any of the
xpath_ functions available in xml2.  I challenge anyone to claim that
the [local-name()="foo] variant is easier to read and less error prone
than the second, namespace-prefixed variant.  They are exactly
equivalent, but the second (quite obviously) is Better(tm).

I understand that XML support is planned and at least partially
implemented for 8.3, but many production instances will be unable (or,
in fact, unwilling) to upgrade to 8.3 for quite some time.  Because
this patch is completely backward compatible it can (theoretically) be
included in future 8.1 and 8.2 releases, and for those of us that need
more full XML support in the short term the upgrade of a contrib
module is probably a very viable option -- it is for me, anyway.

So, to sum up, please let me know what I can do to increase the
chances of getting this patch included.  Alternatively, if my patch is
being vetoed, please let me know that too so that I can create a local
maintenance plan for this.

Thanks in advance.  I've attached the patch again for reference.

--
Mike Rylander
mrylander@gmail.com
GPLS -- PINES Development
Database Developer
http://open-ils.org